Revisited Firebird's keywords set

Revisited Firebird's keywords set
Several keywords were missing, and various words were wrongly included
in the set. I took the current list of keywords out of "keywords.cpp", filtering
out effective reserverd words with a simple script that created a dummy table
with a field named after each word.
While this fixes a few tests (for example, those creating a table with a "start"
field, a Firebird reserverd words not previously registered as such), it may
introduce a backward incompatibility with previous SA releases: should this be
not wanted, I will add previous non-reserved-words to the set.